Amplifying apparatus



Aug. 11,1942. A. aAR'co 2,292,919

" AMPLIFYING APPARATUS Filed Oct. 7, 1938 s Sheets-Sheet 1 INVEN TOR.

ALLEN BAR CO Aug. 11, A BARCQ AMPLIFYING APPARATUS 3 Sheets-Sheet 2 Filed Oct. 7, 19 38 Q/MPEDANCE U P w m E M P b v I v II 1 SHADING INIJl/T 1 INVENTOR.

ALLEN BARC'O ATTORNEY.

Patented Aug. 11, 1942 UNITED STATES PATENTOFFICE AMPLIFYING APPARATUS Allen Barco, Tackson Heights, Long Island, N. Y, assignor to Radio Corporation of America, a corporation of Delaware Application October 7, 1938, Serial No. 233,870

'z Claims. (01. 178- 12) My invention relates to amplifying apparatus, and more particularly to such apparatus as is used for the amplification of video signals generated in a television scanning tube, one type of which is the Iconoscope.

Previously, the amplifiers which have been used in this connection have had several disadvantages. Thermal activity causes the generation of undesired or background noise, and it has been necessary to sacrifice either the frequency 'response characteristics of the amplifier, or if a desirable frequency response was maintained, there has resulted a high noise-to-signal'ratio at the output of the amplifier. Accordingly, it is one of the objects of my invention to provide an amplifier for an Iconoscope which will have a high signal-to-noise ratio. r

In addition to the desirability of obtaining a high signal-to-noise ratio, the frequency response characteristics of the Iconoscope with regard to the signal voltages developed should beas nearly fiat as possible. Heretofore, where an effort has been made to obtain ahigh signalto-noise ratio, this has been done at the expense of the frequency response of the apparatus. Accordingly, it. is another of the objects of this invention to provide an apparatus wherein not only will a high signal-to-noise ratio be obtained, but in which the frequencycharacteristics of the apparatus will suffer substantially none or very little mutilation.

One of the causes of background noise has been due to thermal agitation in the elements involved in the amplifier, and heretofore the effect of noise developed by this phenomenon has been of a comparatively high degree. Accordingly, it is another of the objects of my invention to provide an amplifier in whichthe effect of noise due to thermal agitation may be reduced to a presently known minimum.

The Iconoscope itself contains a collector electrode for collecting electrons emitted from the photoelectric mosaic. This collector electrode has a very definite function in preventing unequal effective light distribution on the photoelectric mosaic per se. Accordingly, it is another objectof this invention to provide an apparatus whereby the collector action of said collector electrode'may be desirably enhanced.

A portion of the mutilation of the frequency response characteristic in an Iconoscope, and consequently its signal developing power, has

This input capacitance acts with the shunt capacitance of the signal voltage developing means to cut down the generation of signal voltages to a great extent particularly, at the higher frequencies. Accordingly, it is another of the objects of my invention to provide an amplifier in which the apparent input capacitance of the stage coupled to the video signal voltage developing means is considerably lessened .or cut down. a

In addition to the action of the aforementioned input capacitance there is also the capacitance of the signal plate in the Iconoscope to ground, or with respect to the reference potential to which the cathodes of the device are usually associated. The capacity distribution around the signal plate itself plays a part in the successful generation of voltages which are indicative of the optical values impinging upon any section of the mosaic. Accordingly, it is another of the objects of my invention to provide a device whereby a more preferable capacity distribution around the signal plate per se is obtained.

In the present known Iconoscopes, there are always developed in the signal plate undesired spurious electrical disturbances. If the attempt is made to compensate for these disturbances after they have been passed through the amplifier along with the amplified video signal, it will be found that such compensatory action is combeen due to the comparatively high input cav paratively unsuccessful. There is usually provided what is termed a shading signal which is impressed out of phase with the spurious signal in an effort to buck out or compensate for the spurious signal. Heretofore, this has not been successful where the attempt has been made to buck out the spurious signal after the video signal has been amplified to any degree, and usually the greater the amplification usually the greater mutilation of contrast in the picture. Accordingly, it is another of the objects of my invention to provide a device whereby the shading signals are inserted at such a position in the system that the undesired or spurious components of signalsare substantially compensated or bucked out.

, Also, heretofore where a good frequency response was maintained at the expense of the sig-' nal-to-noise ratio, noises in the lower frequency spectrum' were amplified considerably. By correcting a highly deficientirequency response at a subsequent stage, such as is done in this invention, the overall gain of the amplifier is very low in the low frequency spectrum and, therefore,

the eifect'of microphonics is greatly minimized. Accprdingly, it is another of the objects of my invention to provide an apparatus in which the effect of microphonics is greatly minimized.

Accordingly, included among the objects of my invention are i 1. To provide an apparatus of the nature described for developing a high signal-to-noise ratio.

2. To maintain a desirable frequency response characteristic of the system.

3. To decrease the effect of background noise due to thermal agitation.

4. To obtain a good collector action within the Iconoscope per se.

5; To decrease considerably the apparent input capacitance of the first stage of the amplifier without the use of excessively large blocking concoupled to the video signal voltage developingmeans.

6. To obtain a desirable capacity distribution around the signal plate of th Iconoscope."

7. To insert a shading signal at such a point in the system as to substantially nullify the action of spurious signals without affecting contrast in the picture per se.

8. To minimize the effect of microphonics in an amplifier. 4

9. Various other purposes and advantages of the present invention may become apparent to those-skilled in the art from a reading of the following specification and claims.

The provisions of my device in general are as follows: Under normal operating conditions, the Iconoscope output current is of the order of a fraction of a microampere. It is desirable to rai e the signal voltage to a level of about onehalf volt (peak-to-peak video signal) before the signal is subjected to subsequent mixin clipping and transmission processes. The output impedance of the Iconoscope is .of such high value that for all practical purposes, the device maybe treated as a constant current device. Accordingly, in myapparatus the signal is impressed on a high resistance whichis coupledto the input circuit of the first stage of the amplifier. The effect of the shunt capacitance of the resistor, the input capacitance of the first stage of the amplifier, and other capacity such as the signal plate to ground capacity is such that with the use of a high resistor the frequency characteristic is strongly affected. In order to obviate the effects of thermal agitation and these capacities, an ar rangement is used whereby the apparent input capacity of the first stage of th amplifier is reduced to a very little degree.

In order to effect a better capacity distribution around the signal plate, an image of thin wires surrounds this plate, and the image is maintained at a potential approximately equal to that of the signal plate by connecting the cage to the cathode of the first amplifier tube.

The shading signals are inserted directly at the output circuit of the Iconoscope" to buck out the spurious or undesired signals before any amplification occurs in order that the contrast of the picture shall not be affected.

The second stage of the amplifier employs a shunt peaking arrangement to extend the frequency characteristic of the apparatus. The second stage feeds directly into the third stage, the output circuit of the latter having desirable frequency characteristics. This stage is arranged so that the frequency characteristic which has been sacrificed in its desirable features by the use of a highly resistive memb r in densers or other undesirable means. For this purpose, there is provided a cathode loaded amplifier, or degenerative amplifier, which provides an output of very low impedance without the sacrifice of amplification,

My invention will best be understood by reference to the figures in which Fig. 1 shows a coupling arrangement between the Iconoscope" and the first stage of the amplifier.

Fig. 2 shows a steadily biased cathode loaded amplifier arrangement similar to Fig. 1.

Fig. 3 is a modification of Fig. 2 with a selfbiasing arrangement.

Fig. 4 shows an alternative form wherein the Iconoscope collector ring is maintained substantially cathode potential of the first stage.

Fig. 5 schematically shows a shield member.

Fig. 6 is the arrangement of Fig. 4 with the circuits are the most important parts of this.

amplifier for it is here that the ultimate limits of signal-to-noise ratio and frequency response are almost wholly determined. It is impossible to remove noise once it has been mixed with video signal. Hence, it is desirable that the video signal developed .be as large as possible in the early stages, and to bring about amplification of the signal with the introduction of a minimum of noise.

Referring toFig. 1, there is shown schematically a method of coupling the Iconoscope" to the first stage of an amplifier. An Iconoscope tube l'contains a signal collecting plate 2 adjacent which is positioned a collector ring 3. It will be appreciated that in these tubes a photoelectric mosaic is positioned adjacent the signal plate tube, but the latter is referred to as such because it is the variations in current in this plate which are used to develop voltages representative of the video signals. A high resistor 3' is connected in series with the signal plate, and the potentials developed across this resistor 3 are impressed on the grid of the amplifying tube 4 through a condenser 5, the grid being connected back to cathode through a resistor 6 and a sourceof steady biasing potential 1. It is in this stage that as high as possible a ratio should be obtained between the signal generated and the noise. a

The problem of obtaining the desired frequency response characteristic for the amplifier permits of a more simple solution because it is possible to alter the frequency response in the early stages of the amplifier, and then by means of proper correction restore the desired frequency characteristic in some subsequent stag For example, let us assume that the video band width will extend from say 60 cycles to a point, which for purposes of illustration will be assumed to be five megacycles. The upper band limit occurs at the frequency at which the shunt resistance and capacity or .reactance which form the load circuit for the'tube are of equal magnitude. For example, with an Iconoscope output capacitance of micromicrofarads, and assuming the input capacitance of a tube such as actually used to be 12 micromicrofarads, and the addition of a few micromicrofarads for stray circuit capacitance brings the total shunt capacity in the neighborhood of micromicrofarads. it may be shown mathematically that in order to achieve this arbitrary band width the load resistance should be in the neighborhood of 1270 ohms.

This low value of load resistor would be satisfactory from a standpoint of frequency response but from the standpoint of signal-to-noise ratio would be highly undesirable. This is true because the noise generated inthe low frequency range by thermal agitation in the load resistor varies as the square root of the resistance, while the signal voltage increases directly as the load resistance, if we should assume the Iconoscope, to be substantially a constant current device and this assumption is perfectly warranted. However, in the upperfrequency range, conditions are different. Here the output voltage of the Iconoscope is determined almost entirely by the load circuit capacity whichalso shunts the thermal agitation potentials developed in the load resistor. If the output capacity and the output resistor are converted to their effective series values, the series resistance thus obtained can be used to calculate directly the thermal agitation potentials which are impressed on the grid-of the amplifier shown. This effective series resistance decreases as the physical shunt resistance is increased, thus causing an improvement in signal-to-noise ratio insofar as noise voltages from the load circuit resistor are concerned. However, increasing tha resistor 'provides the added advantage in that not only does it decrease the effect of thermal noise developed in the resistor but gives agreater signal, and

hence a better ratio of signal to tube noise. The

improvement obtained by this means is particularly beneficial in the low frequency range.

Referring to Fig. 2, there is shown an alternative form of the arrangement of Fig. 1, and in which like symbols refer to like parts. In this instance, an additional resistor 8 is placed in the cathode circuit and hence forms an un-bypassed cathode load. The effect of this naturally is degenerative. It may be shown therefore that this in effect reduces the apparent-input capacitance of the tube 4.

Referring to Fig. 3, there is shown a direct connection between the signal plate 2 and the grid of the first stage amplifying tube 4. In this case, the collector ring 3 is biased slightly with respect to the signal plate 2 by the potentiometer arrangement shown in connection with cathode resistor 8. The arrangement as shown eliminates the steady bias I as shown in Figs. 1 and 2, and provides a self-biasing arrangement instead.

The removal of the coupling condenser 5 from the circuit shown in Figs. 1 and 2 reduces the stray circuit impedance.

It has been found advantageous to maintain the Iconoscope collector ring at a potential nearly equal to but slightly positive with respect to the signal plate. It is convenient to obtain this potential from the cathode of the first amplifier tube from a suitable filter and, accord ingly, Fig. 4 illustrates such an arrangement.

Inthis figure, the signal plate Zis directly connected to the grid of a thermionic tube 4, the latter being joined to the cathode resistor 8 through the series resistor 6, thus making the cathode resistor in effect a potentiometer. The collector ring 3 is connected to the cathode of tube 4 through a resistance 9 and the lower extremity of the resistance 8 is joined to the collector ring through the condenser l0.

Referring to Fig. 5, there is shown the shield arrangement which is placed adjacent the sig- 'nal plate for providing a reduction in the capacities between the signal plate and ground. The

electrostatic shield comprises the individual wire members l5 forming a case which is held by ring members It. Measurement has revealed that the Iconoscope output capacitance consists of two parts. The first comprised the direct internal capacitance between the signal plate and the collector, and has been of the order of 5 micromicrofarads. The second part has been the capacitance between the signal plate and the shielding case in which the Iconoscope is housed. This has also been in the neighborhood .of about 5 micromicrofarads.

instance, has revealed that the cause of the apparent reduction in input capacitance to the amplifying tube of the first stage of the amplifier lies in the fact that the cathode signal voltage has almost the same amplitude and phase as the grid signal voltage.

Referring to Fig. 6, there is shown a first stage circuit arrangement wherein the collector plate has the wire cage positioned adjacent to it. The arrangement is essentially that of Fig. 4 with the exception that the cage member I! is maintained substantially at cathode potential. Hence the signal plate and its surrounding electrostatic shield are both placed very nearly at the same potential. Hence, any capacitative current which would tend to flow would be reduced by the ratio of the grid-cathode-voltage to the signal voltage. This very materially reduces the effective capacitance between the signal plate and ground.

, .Referring to Fig. '7, there is shown the first stage of the amplifier wherein provision is made for the impressing of signals for the purpose of compensating for spurious disturbances. The need for so-called shading signals arises from the fact that the Iconoscope has a characteristric inherently due to its principle of operation of having appear in its output circuit a number of spurious signals. In addition to the desired video signal, it is the purpose of the shading signals to neutralize or buck-out the undesired spurious signals. Since the spurious tend to introduce undesirable phase shifts.

control grid of this tube 34 is connected tothe and resultant loss of picture contrast which might occur if shading signals were inserted.

high resistor, 5 megohms being exemplary. Thus must be done in order to make the shading signals impressed on the first amplifier gridappear as being derived from a constant current source or expressed in other ways, the source of shading signal must not act as an appreciable shunt upon the Iconoscope load.

Referring to Fig. 8, there is shown a complete amplifier circuit. In this figure, the Iconoscope 20 has its signal plate 2 l surrounded by an electrostatic shield memberor cage 22,, and positioned adjacent thereto is a collector ring 23. Connected in series with the signal plate are a pair of resistors 24 and 25, the latter being grounded at the extremity opposite its connection to the signal developing plate. Also connected to the signal plate through a resistor 26 is the control grid of the first stage amplifying tube 21, the tube actually used being one of the type known in the art as the RCA 1851 pentode tube. The cathode of the tube 21 is loaded through a load resistor 28, the latter having the extremity opposite that connected to the cathode of the tube 21 grounded. The control grid is connected back to the cathode load resistor 28 through the resistor 26 and another resistor 29. The static shield 22 is connected back to the cathode of the tube 21, and is also connected through a resistor 30 to the collector ring 23 of the Iconoscope, the latter being grounded through a condenser 31. Theresistor 25 forms the input for the shading signals. The theory back of this arrangement has been explained hereinbefore with reference to Figs. 1 through 7.

inductance 42-. The plate is then grounded through the members 4|, 42 and condenser 43 on one hand and on the other hand through condenser 44, resistor 45, and the biasing battery 33. The signal variations in the plate circuit are then impressed onto the control grid of the tube 46 which is the third stage amplifying tube. This stage is unique with respect to its plate load and method of coupling into the fourth stage. The plate circuit of tube 46 which in this case is a pentode, is connected through the primary of one section 41 of a bifilar winding and is grounded through the bifilar winding, a condenser 46 and a resistance 49. The plate is also grounded through the resistances 50 and 61, andcondenser 66. The screen grid of the tube 46 is connected to ground through condenser 52. The other winding 53 of the bifilar winding is connected to theterminal of the condenser 48 opposite that to which the winding 41 is connected, and the winding 53 is joined to the control grid of the fourth stage amplifying tube 54 through a, condenser 55, the control grid being grounded Investigation of tube noises has shown that I some measure of reduction of tube noise is realized by operating the first stage amplifier as a degenerative triode rather than a pentode. This means that the screen grid bypass 32 should be returned to ground rather than to cathode. While this practice results in a slight increase in input impedance, it affords about 30% reduction in overall nois voltage. The efl'ect of the un desired increase in input capacitance upon frequency response may be easily compensated in the third stage.

The second stage of the amplifier consists of a pentode amplifier employing shunt peaking to extend the frequency characteristic. Battery bias 33 is used to eliminate the need for cathode by-passing or other de-coupling means which The The output of the tube 34 employs a shunt peaking circuit comprising a resistance 41 and an The operational theory upon which this stage is considered to be based is as follows: Returning to the Iconoscope load circuit, it will be found that this load consistseffectively oi. eight micromicrofarads and 300,000 ohms in shunt therewith in actual practice. Assuming the "Iconoscope to simulate a constant current sourceand that the composite gain of the first and second stages of the amplifier is of a magnitude a and is constant throughout the video band, the output characteristic of the second stage may be represented by RIXL E=I 1 A Rl+m where E equals output voltage of the second stage, and I equals "Iconoscope output current, R1 equals Iconoscope load resistance (effective), 01 equals "Iconoscope load capacitance (effective). Since A and I are assumed constant, they may be denoted by lumped constant K. After due simplification, the output characteristic may be seen to be of the form The output obviously varies in phase and amplitude as a function of frequency. Now, if a third stage were used having a plate load of the form R2+JwL2, it may be seen that the gain over these stages is given by where K is a new constant taken to include the mutual conductance of athird stage. Simplification of this expression gives Thus, it may be seen that if the ratio of L2 to R: a

be equal to the product of 01 with R1, the output will be of constant amplitude and without phase' capacity value shunting the load circuit. This means that the value of the reactance of L2 should be small compared to the value of the shunting capacity reactance at the highest frequency to be. amplified, that is to say, the resonant frequency of the composite plate load should fall well above the upper end of the video range.

- series with the load as illustrated in Figure 9.

In actual practice this condition might be fulfilled by making In with microhenries and R2=6.25 ohms. In this case resonance occurs at 8.2 megacycles. Reference should be had to the schematic showing of Figure 10 which, in a simplified form will illustrate the arrangement.

v The output circuit of the third stage feeds, as

shown, into the grid cathode circuit of the fourth In conventional plate supplies the output impedance has a capacity reactance of about 50 ohms so that at the normal 60 cycle alternating current supply, although a regulated supply offers a much lower impedance the situation is still undesirable because. the, regulated supply output impedance varies both in magnitude and phase with respect to frequency, and may at times be negative in nature. Even if the suppli, had negligible impedance, the leads from power supply to the camera may have appreciable reactance. The most satisfactory solution of the problem lies in making the load impedance of the form of R2+JwM2 where M is the mutual inductance between the two windings illustrated in Fig. 11. It is impossible by means of a load impedance of the form R2+JwM2 to eliminate the undesirable eifect of power supply impedance by 'means of the circuit shown in Figure 11.

Referring to Fig. 11, which is illustrative of the output of the third stage of the amplifier shown positive plate potential is impressed through re- The fifth stage tube 64 is unique in its arrangement. In most practical applications, it is desirable to locate this, amplifier which is usually termed the pre-amplifier directly beneath the Iconoscope in order'to secure short input leads. In such cases, the output lead may range from 5 feet to 50 feet or in special instances even longer. It is convenient to have the output lead take the form of a concentric cable a portion of which may be flexible. couple into .the use of this cable without having -to resort to excessively large blocking condensers or other undesirable coupling means which is usually necessitated by such low impedance lines.

Therefore, a cathode loaded or degenerative amplifier stage is used for the purpose of obtain- It is also desirable to be able to ing a low output impedance, and also to keep the cable near ground potential regarding direct current. Also the reduced input capacitance characteristic of the cathode loaded stage is advantageous in that it permits greater gain in the preceding stage.

Accordingly, the said tube 64 is cathode loaded by a resistance 65 and the output of the amplifier plate of the tube BI is connected to ground through a condenser 66 and is joined to the posi' tive side of the plate supply potential through a resistor 61.

Attention has been called hereinbefore to the mathematical explanation "given in connection with the third stage of the amplifier and concerning the frequency characteristic correction stage represented thereby wherein there is shown the effect of the peaking circuit and the power supply impedance. It was pointed out that with a value of L2 equal to 15 microhenries, and R2 equal to 6.5 ohms, resonance would occur at about 8.2

megacycles. Howeventhe value of 6.25 ohms for R2 is diflicult if not impossible to obtain in pracin Figure 8, there is shown the apparatus for overcoming the effect of the power supply impedance. The bifilar winding is used to secure high mutual inductance and the reactance of the windings are made such as to have their-res0nant period, that is to say, the period of resonance with the shunt circuit capacitance fall well outside the video band. The components R1 and C1 are of sufiicient magnitude to eliminate appreciable attenuation or phase shift at low frequencies. The same is true of C3 and R3 which constitute a. conventional grid coupling circuit. The resistor R2 is variable and its initial value may be determined by observation of the picture after the unit is placed in operation. This apparatus forcompensating frequency response has been found to be most desirable regarding ease of adjustment, maximum permissible stage gain and ability to correct large variations in frequency response, in'the present case there being about a 50 to 1 ratio in variation. The idea of correcting a highly deficient frequency response at a subsequent stage has an additional ad- Referring to Figure 12, there is shown schematically the output of the amplifier taken along a concentric cable. In this case, it has been found sufficient in most cases to terminate the cable in its characteristic impedance at the far end only.

It will be realized, of course, that the assumptions made in the case as to such factors as ap-' parent input capacitance, frequency width of the video band and so forth, have been used for purposes of illustration, and slight revision may be is taken between its resistance and ground. The

tice because of the power supply impedance in made as practice in the art changes as, for instance, the actual video band width at the present time is in the neighborhood of 4.5 megacycles rather than 5 megacycles. This, however, does not change to any degree the assumptions made, and the 5 meg'acycle band width has been made for purposes of ease of calculation.

a resistance for connecting the cathode to a point Q eluding a high resistance for applying a source of variable voltage to the control electrode of the discharge path at such intensity and phase relationship as to compensate "for the undesirable spurious signals.

ducing picture signals having a wide range of frequencies in accordance with the scansion thereof, an electron discharge tube including a cathode, anode and at least one control electrode, a point of fixed reference potential, impedance means connected between said cathode and said point of 2. An amplifier for amplifying a wide range of frequencies and wherein the higher frequencies of the range are amplified to a greater extent than the low frequencies, comprising a first amplifier tube having an output electrode, a second amplifier tube having an input electrode, a first and a fixed referencepotential, means connecting the photomosaic output to the control electrode of said electron discharge tube, means including an impedance of relatively high value for connecting the output of said photomosaic to a point on said I first-named impedance means, shielding means intimately associated with theenvelope of thescansion means and embracing the photomosaic thereof whereby similar points on"'said photomosaic have a substantially equal capacity value with respect to said shield member, means consecond mutually inductive winding, means .for'

connecting one end of the first winding to the output electrode of the first amplifier. tube, means for connecting one end'of the second winding to the input electrode of the second amplifier tube, means including a resistance for connecting the other end of the second winding to a point of fixed potential, a condenser for connecting the other end of each of the windings together, and means including a resistance for connecting theother end of the first winding to a. source of positive potential whereby signals in the high frequency portion will be amplified by a greater amount than signals in the low frequency portion of the fre-- quency range.

3. An amplifier for amplifying a wide range of frequencies and wherein the higher frequencies of the range are'to be amplified to a, greater extent than the low frequencies, comprising a first amplifier tube having an-input electrode and an output electrode, a second amplifier tube having an input electrode and an output electrode, a first anda second mutually inductive bi-filar winding, means for connecting one end of the first winding to the output electrode of the first amplifier tube, means including a coupling condenser for connecting one end of the second winding to the input electrode of the second amplifier tube, means including a resistance for connecting the other'end of the second winding to a point of fixed potential, a condenser for inter-connecting the'other end of each of the windings, and means including a resistance for connecting the other end of the first winding to a source of positive potential whereby signals in the high frequency portion will be amplified by, a greater amount than signals in the low frequency portion of the frequency range.

4. An amplifier for television transmission purposes comprising a scanning tube having an'envelope including therein cathode means and photomosaic means, said photomosaic means pronecting the shield member to the cathode of said electron discharge device, said impedance of relatively high value acting to enhance the output voltage developed by the scanning tube in the low frequency range and developing a signal having a high signal-to-noise ratio, ampilfying means coupled to said electron discharge tube for amplifying the signals developed in the output circuit thereof and compensating means connected to said amplifying means, said compensating. means emphasizing the signals in the higher frequency range impressed thereon from said amplifying means whereby the signals developed in the scansion means are materially amplified throughout the frequency range thereof without substantial distortion and whereby a signal having a high signal-to-noise ratio is developed.

5. Apparatus in accordance with claim 4 wherein said shielding means comprises a cage-like- ,member comprised of a plurality of wire members intimately embracing the outer portion of the envelope of the scansion means.

6. Apparatus in accordance with claim 4 wherein said compensating means comprises a first thermionic tube having anode, cathode and at least one control electrode, impedance means ing the output thereof derived from a cathode load.

ALLEN BARCO. 

